1. Field of the Invention
The present invention generally relates to a focus servo controlling apparatus, an information reproducing apparatus and an information recording apparatus including such a focus servo controlling apparatus. More particularly, the present invention relates to a focus servo controlling apparatus for controlling a focal point of a light beam when optically recording or reproducing information with respect to an information record surface of an information record medium, and an information reproducing apparatus and an information recording apparatus including such a focus servo controlling apparatus.
2. Description of the Related Art
When optically recording or reproducing information with respect to an information record medium such as an optical disc, it is necessary to make a focal point of a light beam, such as a laser light for recording or reproducing the information, accurately coincident with a position on an information track of an information record surface of the information record medium.
At this time, as a positional control of the focal point, there are a positional control in a direction perpendicular to the information record surface and a positional control in a direction parallel to the information record surface. Among them, the positional control in the direction perpendicular to the information record surface is often performed by a so-called focus servo control.
Here, as a method of performing the focus servo control, there is a so-called astigmatic method or a so-called Foucault method. In either of these methods, a so-called S curve as shown in FIG. 6 is obtained as a focus error signal SFE on the basis of a reflection light of a light beam from an information record medium, and the position of the focal point of the light beam is made coincident with the position of the information record surface by moving an objective lens in a direction perpendicular to the information record surface by an actuator etc., so that the focus error signal SFE becomes xe2x80x9c0xe2x80x9d when a servo loop of a focus servo for this S curve (i.e., the focus error signal SFE) is in a servo close condition.
At this time, the S curve shown in FIG. 6 is obtained as follows. Namely, in case of the focus servo control by means of the astigmatic method for example, the reflection light of the light beam, to which an astigmatism is given, is received or detected by a four divided light detector. Then, a difference between (i) a signal obtained by adding detection signals of two light-detecting portions on one diagonal line of the four divided light detector and (ii) a signal obtained by adding detection signals of two light-detecting portions on another diagonal line of the four divided light detector is calculated. Such an S curve calculated as the difference has an output voltage corresponding to a focus error amount of the focal position with respect to the information record surface.
Since the S curve is generated on the basis of the reflection light of the light beam as described above, it is difficult to obtain an accurate S curve under a circumstance that the intensity of the reflection light cannot be sufficiently obtained.
Therefore, a so-called focus OK signal (which is a signal indicative of an allowance to change the open condition of the focus servo loop to the close condition, and which will be simply referred to as an xe2x80x9cFOK signalxe2x80x9d hereinafter) is generated when the intensity of the reflection light exceeds a predetermined threshold level, and the open condition of the servo loop is changed to the close condition after this FOK signal is actually generated, to thereby obtain the S curve. This predetermined threshold value to generate the FOK signal is fixed after an adjustment at the time of manufacture.
However, in case of optically recording or reproducing the information with respect to the information record medium, an disturbance light such as a light from an external portion of the information recording or reproducing apparatus or a light due to a reflection within the information recording or reproducing apparatus may be irradiated onto the light detecting element, which is supposed to receive or detect the reflection light, in addition to the reflection light to be detected.
The intensity of this disturbance light may change in accordance with a change of a circumstance where the information recording or reproducing apparatus is installed, or may change due to an aged deterioration.
On the other hand, the above mentioned predetermined threshold level is set in advance in consideration with the influence of the disturbance light (i.e., so as to generate the FOK signal normally even in such a condition that the intensity of an expected disturbance light is added on the intensity of an original reflection light). However, if the intensity of the disturbance light changes as mentioned above, there arises a following problem. Namely, if the intensity of the disturbance light decreases, even in such a condition that a sufficient intensity of the reflection light is obtained, the intensity of the total of the reflection light and the disturbance light may not exceed the predetermined threshold value, resulting in that the FOK signal cannot be generated at the timing when the FOK signal is to be generated. Alternatively, if the intensity of the disturbance light increases, even in such a condition that a sufficient intensity of the reflection light is not obtained, the intensity of the total of the reflection light and the disturbance light may exceed the predetermined threshold value, resulting in that the FOK signal is generated at the timing when the FOK signal is not to be generated (e.g., the FOK signal is kept to be generated) or is not generated at the timing when the FOK signal is to be generated.
If there arises the above mentioned problem as for the generation timing of the FOK signal, the open condition of the focus servo loop cannot be changed to the close condition at an appropriate timing, resulting in that it is difficult or impossible to perform an accurate focus servo control, which is a problem.
It is therefore an object of the present invention to provide a focus servo controlling apparatus, which can perform a focus servo control by accurately changing the open condition of the focus servo loop to the close condition even if the intensity of the disturbance light and the intensity of the reflection light change due to the aged deterioration etc., and which enables an operation of accurately recording or reproducing the information optically, and also an information recording apparatus having such a focus servo controlling apparatus, as well as an information reproducing apparatus having such a focus servo controlling apparatus.
The above object of the present invention can be achieved by a focus servo controlling apparatus for performing a focus servo control by changing a condition of a focus servo loop from an open condition to a close condition in correspondence with a level of a reflection light signal, which is obtained as a reflection light of a light beam reflected from an information record medium is detected by a light receiving element such as an optical pickup. The focus servo controlling apparatus is provided with: a disturbance level detecting device such as a servo control circuit for detecting a level of a disturbance light signal, which is obtained as a disturbance light other than the reflection light is received by the light receiving element; and a controlling device such as a system controller for setting a level, which is larger than the level of the disturbance light signal detected by the disturbance level detecting device, as a threshold level and for changing the condition of the focus servo loop from the open condition to the close condition on the basis of a timing when the level of the reflection light signal becomes equal to or larger than the set threshold level.
According to the servo controlling apparatus of the present invention, since the threshold level is variable to be larger than the level of the disturbance light signal and since the focus servo loop is transferred into the close condition on the basis of the timing when the reflection light signal becomes equal to or larger than the threshold level, it is possible to transfer the focus servo loop into the close condition precisely by eliminating the influence of a change in the level of the disturbance light signal even if the level of the disturbance light signal changes.
Accordingly, even if the level of the disturbance light signal changes due to the aged deterioration or the like, it is possible to perform optically reproducing or recording the information accurately by performing the focus servo with precisely transferring the focus servo loop into the close condition.
In one aspect of the focus servo controlling apparatus, the controlling device sets as the threshold level a level obtained by adding an addition level, which is set in advance on the basis of a minimum level of the reflection light signal detected in advance, onto the level of the disturbance light signal detected by the disturbance level detecting device.
According to this aspect, since the level obtained by adding the addition level, which is set in advance on the basis of the minimum level of the reflection light signal, onto the level of the disturbance light signal is set as the threshold level, it is possible to precisely transfer the focus servo loop into the close condition by means of a relatively simplified structure.
In another aspect of the focus servo controlling apparatus, the apparatus is further provided with a minimum level detecting device such as a servo control circuit for detecting a minimum level of the reflection light signal. The controlling device sets the threshold level to be less than the detected minimum level.
According to this aspect, even if the minimum level of the reflection light signal changes while the level of the disturbance light signal changes, it is still possible to precisely and surely transfer the focus servo loop into the close condition.
In another aspect of the focus servo controlling apparatus, the apparatus is further provided with a timing signal generating device such as a servo control circuit for generating a timing signal indicating a timing when the reflection light signal becomes equal to or larger than the set threshold level. The controlling device changes the condition of the focus servo loop from the open condition to the close condition after the timing signal is generated by the timing signal generating device.
According to this aspect, since the focus servo loop is transferred into the close condition on the basis of the timing signal, it is possible to surely transfer the focus servo loop into the close condition.
The above object of the present invention can be also achieved by an information reproducing apparatus provided with: (i) the above described focus servo controlling apparatus of the present invention (including its various aspects): (ii) a light emitting device such as an optical pickup for emitting the light beam onto the information record medium while performing a focus servo control by the focus servo loop in the close condition, and (iii) a reproducing device such as an EFM (Eight to Fourteen Modulation) encoder/decoder for reproducing information recorded on the information record medium on the basis of the reflection light signal.
According to the information reproducing apparatus of the present invention, it is possible to precisely reproduce the information by surely performing the focus servo control with precisely transferring the focus servo loop into the close condition, even if the level of the disturbance light signal changes.
The above object of the present invention can be also achieved by an information recording apparatus provided with: (i) the above described focus servo controlling apparatus of the present invention (including its various aspects); and (ii) a recording device such as an EFM encoder/decoder for emitting the light beam, which corresponds to record information to be recorded, onto the information record medium while performing a focus servo control by the focus servo loop in the close condition, to thereby record the record information onto the information record medium.
According to the information recording apparatus of the present invention, it is possible to precisely record the record information onto the information record medium by surely performing the focus servo control with precisely transferring the focus servo loop into the close condition, even if the level of the disturbance light signal changes.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.